Stimulated Brillouin scattering (SBS) is a non-linear interaction of light (photons) traveling through a transparent medium such as a glass matrix and interacting with spatial and temporal variations in the refractive index of the medium. In optical fibers, SBS occurs when the light interacts coherently with acoustic waves in the fiber via combination of electrostriction and a photo-elastic effect.
SBS causes a change in the frequency of the light traveling through the transparent medium. The change in frequency of the light due to SBS as it propagates through the transparent medium (e.g., an optical fiber) is called the Brillouin frequency shift (BFS) and is denoted herein as Δf.
SBS effects for guided light traveling in optical fibers limit the use of the optical fibers for some applications. For example, it limits the maximum optical power throughput of optical fiber transmission systems. As the input optical power increases above what is known as the threshold power, the amount of optical power that can be transmitted down the optical fiber reaches an upper limit. Any additional input power to the fiber results in SBS wherein the light scatters in the backward direction due to interaction with acoustic phonons rather than propagating in the forward (launch) direction as a higher-power signal.
In fiber-based sensor applications, SBS limits the sensing range. For example, in some fiber-based sensors, the BFS distorts the spectrum at the far end of the sensing fiber, which in turn distorts the sensor measurement. This is problematic, especially since fiber-based sensors are finding increasing use in the remote monitoring of a wide variety of structures such as bridges, oil and gas pipelines, dams, power lines, tunnels, oil wells, etc.